Preparation of copper alkoxides and sol/gel precursors of superconducting ceramics

ABSTRACT

The invention relates to the preparation of high purity, chloride- and alkali metal-free copper (II) alkoxides by means of the reaction of an alcoholic alkali metal alkoxide solution with copper (II) fluoride; ammoniating the resulting solution to render soluble the resulting copper (II) alkoxide; and filtering the resulting solution to obtain an alkali metal- and chloride-free alcoholic copper (II) alkoxide solution. The resulting solution is useful in the preparation of superconducting compound such as yttrium-barium-copper oxide superconductor.

This invention relates to the preparation of ultra-pure copperalkoxides. In particular, this invention relates to a process for thepreparation of impurity-free, especially chloride-free, ultra-purecopper alkoxides suitable for use in the preparation of superconductiveceramic materials.

BACKGROUND OF THE INVENTION

In the field of superconductive ceramic materials, large advances havebeen made since the 1970's, and in particular, since 1985. Whereas adecade or so ago superconductivity was observed only at or near theliquid helium temperature of 1° Kelvin (K), recent advances have raisedsuperconducting temperature to nearly 100° K. These advances have nowbrought superconducting ceramics within the temperature range of liquidnitrogen which results in considerable cost savings. While many factorshave contributed to expanding the temperature range of superconductivematerials, one of the important requirements is the use of materials inthe manufacture of superconducting ceramics which will not leaveundesirable impurities in the final product. Among the more ubiquitousimpurities which might find their way into finished superconductingceramics are carbon, aluminum, silicon and chlorine (as chloride ion).

The earliest of the higher temperature superconducting ceramic materialswere quaternary ceramics composed of yttrium or a lanthanide series rareearth metal, an alkaline earth metal such as barium or strontium, copperand oxygen. More recent discoveries have brought forth five componentsuperconducting ceramics such as thethallium-calcium-barium-copper-oxygen series of materials. While themechanism by which these ceramic materials are superconducting is notfully understood, there is some speculation that the electrons flowingin these superconducting ceramics travel through "tunnels" formed byalternating layers of metal and oxide ions. While the crystal structureof these ceramics is complex and not fully determined, it is known thatthey are layered structures sensitive to moisture, heat and pressure. Itis further known that distortions in the crystal structure can greatlydecrease or completely destroy the superconducting properties of theceramic material. These distortions can be created in the crystal by theinclusion of larger or smaller ions within the crystal lattice. It iseasier to envision such a distortion using a larger ion.

Imagine a large box full of baseballs neatly stacked in layers, one ontop of the other and held in place by the walls of the box. The voidspace between the baseballs would be the "tunnels" through which"electrons" would flow. However, if a basketball were placed in thecenter of the box, the layered order of the baseballs would be locallydestroyed and the flow of "electrons" blocked. Likewise, small marblesplaced in void spaces formed by the baseball layers would block the flowof "electrons." For these reasons, impurities such as carbon, aluminum,silicon and chloride ion are not desirable in a superconducting ceramic.

The reported superconducting ceramics have been prepared by thermalconversion using various inorganic salts such as nitrate, carbonate ormetal-organics such as alkoxide starting materials. Care in preparationof these starting materials will eliminate silicon and other cations asimpurities. Thermal conversion at 800°-1000° C. in an oxygen richatmosphere causes the carbon present in the starting materials to beoxidized to carbon dioxide gas which volatilizes from the ceramicproduct, thereby removing carbon as an impurity. Nitrate decomposes tovolatile nitrogen oxides and likewise escapes from the ceramics. Whilenitrates do not leave impurities in the ceramic product, their usecreates air pollution control problems in a production scale up. Theremoval of chloride, however, cannot so readily be accomplished.

While chloride may not, at first glance, seem to be a problem in theproduction of superconducting materials, in actual fact chloride saltsare frequently used as the base materials from which the carbonates,nitrates and alkoxides are prepared. During these preparations, traceamounts of chloride ion trail along with the carbonate, nitrate oralkoxide product. Removal of the trace chloride may entail numerouspurification steps and be prohibitively expensive. Among the mostdifficult of the raw materials to purify are the copper (II) alkoxidesused in the preparation of superconducting ceramics. The terms copper(II) and cupric refer to copper in the +2 valence state.

The most common method of preparing a copper (II) alkoxide has been thereaction of an alcoholic solution of anhydrous cupric chloride with analcoholic solution of an alkali metal alkoxide prepared in situ byreacting an alkali metal with an excess of an alcohol. While any of thealkali metals may be used, the preferred metal is lithium for safetyreasons. The alcohol is typically one of C₁ to C₄ alcohols, thoughothers may be used when desirable. A typical reaction may be thatbetween lithium metal and anhydrous methanol to produce a methanolicsolution of lithium methoxide. This lithium methoxide solution isfiltered to remove any precipitate that may have formed and the filteredsolution is added to a solution of anhydrous cupric chloride inanhydrous methanol. The lithium methoxide and cupric chloride react toform copper (II) methoxide which precipitates and lithium chloride whichis soluble in methanol. The copper methoxide is collected by filtrationand washed with several methanol washes.

While the reaction of cupric chloride and lithium methoxide seeminglyseparates the copper (II) methoxide from lithium chloride; in fact, someof lithium chloride and also cupric chloride may become entrained in theprecipitate. While washing the precipitated copper (II) methoxide withmethanol may remove some of the chloride compounds, sufficient chloridemay remain to adversely affect superconductivity in the final ceramicproduct. It would be preferable that the copper (II) methoxide remain insolution and that the chloride containing compounds precipitate. Beingaware of the difficulties encountered in the preparation of copper (II)alkoxides, this invention presents a new method for the preparation ofcopper (II) alkoxides uncontaminated by chloride containing materials.Specifically, the invention relates to the preparation of ultra purecopper (II) alkoxides wherein the copper (II) alkoxide is free of anionsas well as any cations which may adversely affect the superconductingproperties of copper containing superconducting ceramics.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to the preparation of high purity copper (II)alkoxides, said copper (II) alkoxides being essentially free ofcontaminating anions or cations which could adversely affect thesuperconducting properties of copper containing superconductingceramics. As used in this description, the terms solution and solutionmixture means a homogeneous solution, or a solution which contains aprecipitate or suspension of some materials. Where necessary the textwill clearly state which is meant. Solutions obtained after filtrationand prior to any subsequent reaction or treatment do not contain aprecipitate.

The method of this invention for the preparation of high purity copper(II) alkoxides comprises the steps of (a) reacting an alkali metal withan anhydrous, deoxygenated C₁ to C₈ alcohol and filtering the resultingreaction product to obtain an anhydrous, deoxygenated alcoholic alkalimetal alkoxide solution, Solution A; (b) adding anhydrous copper (II)fluoride to Solution A to form an alcoholic copper (II) fluoride-alkalimetal alkoxide solution mixture, Solution B; (c) heating Solution B to atemperature in the range of about ambient temperature to about 10° C.below the boiling point of the alcohol being used for a length of timein the range of about 0.25 hour to about 8 hours to form a solutionmixture containing an alkali metal fluoride and a copper (II) alkoxidesolution, Solution C; (d) ammoniating Solution C with anhydrous ammoniato form a saturated ammoniacal alcoholic solution mixture containingammoniacal copper (II) alkoxide in solution and an alkali metal fluorideand cupric fluoride precipitate, Solution D; and (e) filtering SolutionD to remove the alkali metal fluoride and cupric fluoride precipitateand obtain an alkali metal free copper (II) alkoxide solution useful inthe manufacture of superconductive compounds, Solution E. A dry,powdered, anhydrous copper (II) alkoxide that is useful in themanufacture of superconductive compounds can be obtained from Solution Eby evaporation of the ammonia and the excess alcohol.

The ammoniacal, alkali metal free copper (II) alkoxide solution,Solution E, prepared above may be used to prepare a homogeneous sol/gelthat can be used to prepare superconducting ceramics. In the preparationof such a sol/gel, the ammoniacal copper (II) alkoxide solution,Solution E, is first hydrated by the addition of 2 moles of water foreach mole of copper (II) alkoxide present. The solution is stirred frombetween 0.5 hour to about 2 hours to assure that the copper (II) ionsare completely hydrated. The solution pH, during the hydration step,measures above 11. After the hydration has been completed, a hotsolution of barium alkoxide in excess alcohol is added to the hydratedcopper (II) solution. It is desirable that the barium alkoxide solutionbe at temperature greater than 50° C. Thus, though methanolic orethanolic solutions of a barium alkoxide could be used, it is desirableto use one of the higher boiling alcohols such as propanol, isopropanol,butanol, 2-butanol and the like. The amount of barium alkoxide added tothe hydrated copper (II) solution is such that the barium-copper ratiois about 2:3. After the addition of the barium alkoxide to the hydratedcopper (II) solution, the resulting solution mixture is heated attemperature in the range of about 60° C. to about 70° C. for time in therange of about 0.5 hour to about 2.0 hours. The resulting barium-coppersolution is a green color and is somewhat viscous. After the mixing ofthe barium-copper solution is completed, an alcoholic lanthanide seriesmetal alkoxide solution such as a yttrium alkoxide solution, agadolinium alkoxide solution, a lanthanum alkoxide solution and the likeis added to the barium-copper mixture. The amount of the lanthanidemetal alkoxide added is such that the lanthanide metal-barium-copperratio is about 1:2:3. After the addition of the lanthanide metalalkoxide is completed, the resulting solution mixture is mixed andheated to a temperature in the range of about 50° C. to about 60° C. fora time in the range of about 8 hours to about 16 hours. The resultingsolution is a viscous sol. Upon concentrating and ageing the sol, itturns into a sol/gel which can be used to prepare superconductingceramics by methods known in the art. In choosing the alkoxide/alcoholused with the lanthanide metal, the same considerations should be takeninto account as with the choice of the barium alkoxide. While any of theC₃ or higher alcohols may be used to prepare the alcoholic barium andlanthanide metal alkoxides, for economic reasons, isopropanol ispreferred.

The copper (II) alkoxides prepared according to this invention utilizesthe fact that both copper (II) fluoride and lithium fluoride areinsoluble in alcohols. (Handbook of Physics and Chemistry, 48th Ed.,Chemical Rubber Co., 1968). While copper (II) fluoride may be insolublein an alcohol, it is found, nonetheless, to be reactive toward reagentssuch as alkali metal alkoxides. Reaction between copper (II) fluorideand lithium alkoxide in alcohol solution produces an insoluble lithiumfluoride and an insoluble copper (II) alkoxide. However, upon theaddition of ammonia, the copper (II) alkoxide forms an ammoniacal copper(II) alkoxide complex which is soluble in the alcohol whereas lithiumfluoride and copper fluoride do not form such soluble complexes andremain insoluble. Therefore, by ammoniating the reaction mixture andfiltering the resulting ammoniacal solution, an ammoniacal copper (II)alkoxide solution, free of anions or cations which may adversely affectsuperconductivity, can be obtained. The resulting filtered solution maybe used as is for the preparation of superconducting ceramics by methodssuch as the sol-gel method, or the alcohol and ammonia may be removed invacuo, by heating or by a combination thereof to produce a dry, solidcopper (II) alkoxide product which can be used in the preparation ofsuperconducting ceramics.

The alkali metals used in the present invention may be selected from thegroup consisting of lithium, sodium or potassium, though lithium ispreferred because it is the safest to use. The alcohol used to preparethe copper (II) alkoxide of the present invention may be selected fromthe group consisting of linear, branched or cyclic aliphatic C₁ to C₈alcohols such as methanol, ethanol, propanol, isopropanol, butanol,octanol, cyclohexanol and the like. The preferred alcohol is methanol.

The examples which follow are for illustration purposes and are notmeant to be limiting. Standard dry box and inert atmosphere techniqueswere used throughout the preparation of the copper (II) alkoxides ofthis invention.

EXAMPLE I

A 0.625 g sample of clean lithium metal was reacted with 300 ml ofdeoxygenated, anhydrous methanol under an inert atmosphere and filteredto remove any precipitated material. A 4.57 g sample of pure whiteanhydrous copper (II) fluoride was added to the filtered lithiummethoxide solution and the resulting solution mixture was heated to45°-50° C. for 0.5 hour to facilitate the formation of copper (II)methoxide which formed as a blue-grey precipitate. After cooling andallowing the reaction solution mixture to stand at ambient temperaturefor about 2.0 hours, the solution mixture was ammoniated by passinganhydrous ammonia through the solution. The solution turned a very darkblue upon the addition of ammonia, thereby indicating the complexing ofcopper (II) methoxide by ammonia. The resulting ammoniacal solution wasfiltered to give a clear dark blue solution and a precipitate of lithiumfluoride and unreacted copper (II) fluoride. The resulting clear, darkblue, ammoniacal copper (II) methoxide solution was used as is for thepreparation of a yttrium-barium-copper-oxygen superconducting ceramicaccording to known art.

EXAMPLE II

The same procedure as Example I was followed to obtain a clear, darkblue, ammoniacal copper (II) methoxide solution after filtration andremoval of lithium fluoride and copper (II) fluoride. This solution wasevaporated to dryness in vacuo. As the ammonia and methanol were removedfrom the solution, copper (II) methoxide precipitated as a blue-greypowder. A sample of the powder was hydrolyzed and tested for thepresence of lithium by the lithium flame test. No lithium was detected.The remaining powder was calcined and used to manufacture ayttrium-barium-copper-oxygen superconducting ceramic according to knownart.

EXAMPLE III

A homogeneous sol/gel useful for the preparation of superconductingceramics was prepared in the following manner:

(a) Two moles of the triply-distilled water were added in a dropwisemanner to 1 mole of a 0.21 Molar ammoniacal copper (II) methoxidesolution prepared according to Example I. The pH of the resultinghydrated ammoniacal copper (II) methoxide solution was greater than 11.The resulting clear blue solution was stirred at ambient temperature forabout 1 hour to ensure complete hydration of the copper (II) methoxide.

(b) A hot solution of barium isopropoxide, temperature greater than 53°C., was then added to the hydrated ammoniacal copper (II) methoxideprepared in (a). The barium-copper ratio of the resulting solution wasabout 2:3. The resulting barium-copper solution was mixed for about 1hour at a temperature in range of 60° C. to about 70° C. to yield agreen, moderately viscous barium-copper solution.

(c) To the barium-copper solution resulting from (b), a solution ofyttrium isopropoxide in excess isopropanol was added in a dropwisemanner. The resulting yttrium-barium-copper solution, which had ayttrium-barium-copper ratio of about 1:2:3, was stirred for a time inthe range of 1 hours to about 4 hours at a temperature in the range offrom about 50° C. to about 60° C. The resulting yttrium-barium-coppersolution was a viscous sol which was concentrated and aged underconditions known in the art.

The sol/gel of (c) was used to prepare a yttrium-barium-copper-oxygensuperconducting ceramic by thermal conversion methods known in the art.

EXAMPLE IV

A homogeneous sol/gel useful for the preparation of superconductingceramic was prepared according to Example III, the difference being thatyttrium isopropoxide was replaced with lanthanum isopropoxide. Thesol/gel so prepared was used to prepare a lanthanum-barium-copper-oxygensuperconducting ceramic by thermal conversion methods known in the art.

What is claimed is:
 1. A method for the preparation of high puritycopper (II) alkoxides comprising:(a) reacting an alkali metal with ananhydrous, deoxygenated alcohol and recovering from the resultingreaction product an anhydrous, deoxygenated alcoholic alkali metalalkoxide solution; (b) admixing anhydrous copper (II) fluoride with thealkali metal alkoxide solution; (c) heating said mixture to atemperature of between from about ambient temperature and about 10° C.below the boiling point of the alcohol for between about 0.25 hour andabout 8 hours to form a solution of alkali metal fluoride and copper(II) alkoxide; (d) ammoniating said alkali metal fluoride and copper(II) solution with anhydrous ammonia to form a saturated ammoniacalalcoholic solution of ammoniacal copper (II) alkoxide in solution and analkali metal fluoride precipitate; and (e) recovering an alkali metalfree ammoniacal copper (II) alkoxide solution.
 2. A method in accordancewith claim 1 wherein the anhydrous alcohol is a linear, branched orcyclic C₁ to C₈ anhydrous aliphatic alcohol.
 3. A method in accordancewith claim 1 wherein the anhydrous alcohol is anhydrous methanol.
 4. Amethod in accordance with claim 1 wherein the alkali metal is selectedfrom the group consisting of lithium, sodium and potassium.
 5. A methodin accordance with claim 4 wherein the alkali metal is lithium.
 6. Amethod for the preparation of a homogeneous sol/gel useful in thepreparation of superconducting ceramics comprising:(a) reacting analkali metal with an anhydrous deoxygenated alcohol and recovering fromthe resulting reaction product an anhydrous, deoxygenated alcoholicalkali metal alkoxide solution; (b) admixing anhydrous copper (II)fluoride with the alkali metal alkoxide solution; (c) heating saidmixture to a temperature of between about ambient temperature and about10° below the boiling point of the alcohol for between about 0.25 hourand about 8 hours to form a solution of alkali metal fluoride and acopper (II) alkoxide; (d) ammoniating said alkali metal fluoride andcopper (II) alkoxide solution with anhydrous ammonia to form a saturatedammoniacal alcoholic solution of ammoniacal copper (II) alkoxide insolution and an alkali metal fluoride precipitate; (e) recovering analkali metal free ammoniacal copper (II) alkoxide solution; (f)hydrating said ammoniacal copper (II) alkoxide solution by the additionof 2 moles of triply distilled deionized water for each such mole ofammoniacal copper (II) alkoxide present in said solution and stirringsaid water containing solution for a time in the range of about 0.5 hourto about 2 hours; (g) adding a solution of barium alkoxide in excessalcohol, at a temperature of at least 55° C., to the ammoniacal copper(II) alkoxide solution obtained in (f), said amount of barium addedbeing about 2/3 the amount, on a molar basis, of copper present andstirring the resulting barium-copper containing solution at atemperature from about 60° C. to about 70° C. for a time in the range ofabout 0.5 hour to about 2.0 hours; (h) adding a lanthanide series metalalkoxide solution to the solution resulting from (g), the amount oflanthanide metal added being about 1/3, on a molar basis, the amount ofcopper present in (g) and heating the resulting lanthanidemetal-barium-copper solution at a temperature in the range of about 50°C. to about 60° C. for a time in the range of about 8 hours to about 16hours; and (i) concentrating and ageing the lanthanidemetal-barium-copper solution to produce a sol/gel.
 7. A method inaccordance with claim 6 wherein the anhydrous alcohol used to preparethe copper (II) alkoxide is a linear, branched or cyclic C₁ to C₈aliphatic alcohol.
 8. A method in accordance with claim 6 wherein theanhydrous alcohol used to prepare the copper (II) alkoxide is methanol.9. A method in accordance with claim 6 wherein alkoxide ion of thebarium and lanthanide series metal alkoxides is a linear, branched orcyclic C₃ to C₈ aliphatic alkoxide.
 10. A method in accordance withclaim 6 where the barium and lanthanide series metal alkoxides areisopropoxides.
 11. A method in accordance with claim 6 wherein thelanthanide series metal is yttrium.
 12. A method in accordance withclaim 6 wherein the alkali metal is selected from the group consistingof lithium, sodium and potassium.
 13. A method in accordance with claim6 wherein the alkali metal is lithium.